1. Field of the Invention
The present invention relates to image data processing for correcting jaggies, discontinuities and satellite dots at characters and thin lines in an image.
2. Description of the Related Art
Various jaggies appear at edges of characters and line images, which are produced by binarization of multivalued images.
For example, Japanese Patent Publication No. 60-53982 (53982/1985) has disclosed the following technique in connection with jaggy correction for binary images. In this technique, binary signals representing black and white and produced from image signals are not immediately encoded. Alternatively, when a situation of signal generation for a certain section or period of the binary signals satisfies predetermined conditions with respect to the primary scanning direction or subscanning direction, predetermined portions of the signals within the above section are corrected from the white signal to the black signal or vice versa, whereby it is possible to prevent encoding of complicated border lines of the image without another processing before transmission. Based on assumption that, for example, the probability of existence of characters and thin lines having one picture element width is low, a series of signals may include continuous black signals for two or more picture elements, which are followed by a white signal for only one picture element further followed by continuous black signals for two or more picture elements. In this case, only the white signal for one picture element is corrected into a black signal. Alternatively, In the case where continuous white signals for two or more picture elements are followed by a black signal for only one picture element, which are further followed by continuous white signals for two or more picture elements, only the black signal for one picture element is corrected into the white signal. In the case where black and white signals each for one picture element alternately appear one by one, all the signals in each alternating section are corrected into black or white signals. In these manners, jaggies on binary images and satellite dots are corrected and removed.
Here, the satellite dots are dots which are present near edges of images (generally, within a range of about 500 .mu.m from edges).
However, the binary image processing is performed with respect to only the main scanning direction and subscanning direction, so that jaggies appearing at directions other than the above cannot be corrected.
The following means for correcting jaggies have generally been known:
(1) Noise removal filtering with a fixed nonlinear filter.
(2) Filtering with pattern matching.
(3) Noise removal filtering with tracing in an edge direction.
In the above measures (1), the nonlinear filter is effective to noises appearing unsteadily and therefore is suitable to jaggies on images. However, since the measures (1) use a preset fixed filter, it is impossible to remove jaggies, discontinuities and satellite dots appearing aperiodically in various forms. The measures (2) and (3) employ filters which are variable with respect to local images to be noted or processed. However, the measures (2) require setting of a large number of patterns for processing various forms, and therefore are not practical. The measures (3) cannot easily perform labeling, and therefore are not practical.